(1) Field of the Invention
This invention relates to a combustion catalyst for removing organic compounds from a gas stream containing the same, and a process for removing organic compounds from the gas stream using the catalyst. More particularly, it relates to such a combustion catalyst as are effective for removing organic compounds diluted in a gas stream, and a process for removing such organic compounds from the gas stream using the catalyst.
(2) Description of the Related Art
A wide range of synthetic organic compounds are processed and produced as intermediates and products in chemical plants, with a concomitant leakage or accidental discharge into environment. Frequently, exhaust gases from waste incinerator plants contain organic compounds. Many such organic compounds are harmful to both the environment and health. Consequently, there is a need to establish an effective emission control technique. Particularly, there is a need to develop effective and practical techniques for controlling emission of volatile organic compounds or halogen-containing organic compounds,
There have been proposed many methods for removing organic compounds from gas streams, including adsorption, direct combustion and catalytic combustion methods. Though the adsorption method may work effectively when it is employed for treating a gas stream having a high content of organic compounds. Removal efficiency may deteriorate when it is used for treating a gas stream containing dilute organics. In the case of the direct combustion method, a temperature as high as 800xc2x0 C. or higher is required, which is not economically viable. Furthermore, the direct combustion method suffers from the problem that nitrogen oxides are generated which could result in a source of secondary pollution.
As a catalytic combustion method, for example, Japanese Patent Public Disclosure (KOKAI) (hereinafter referred to xe2x80x9cJP-Axe2x80x9d) No. HEI-4-250825 describes a method for treating a gas stream containing halogen-containing organic compounds wherein said gas stream is brought into contact with an acidic zeolite. In the method, the halogen-containing organic compounds are treated by contacting the gas stream with an acidic zeolite or an acidic zeolite that is loaded and/or exchanged with at least one metal belonging to the second to sixth periods of the Periodic Table of Elements.
JP-A HEI-4-284849 discloses a catalyst decomposing 1,2-dichloroethane and a method for treating a waste gas stream with such a catalyst. In the disclosed method, the 1,2-dichloroethane decomposing catalyst comprises an acidic zeolite or an acidic zeolite which has been loaded and/or ion-exchanged with a transition metal or metals.
JP-A HEI-8-38896 discloses a catalyst for decomposing chlorinated volatile organic compounds. According to the disclosure, the catalyst is capable of decomposing chlorinated volatile organic compounds in the presence of steam and oxygen. The catalyst comprises at least one selected from platinum, palladium and ruthenium as a primary catalytically active substance which is supported on a carrier consisting essentially of zirconia, said catalyst supporting further a co-catalyst comprising boron oxide. In the catalyst, the proportion of the primary catalytically active substance is, as calculated as metal, 0.1 to 5% by weight on the basis of the total weight of catalyst, and the proportion of the co-catalyst is, as calculated as B2O3, 2 to 5% by weight on the same basis.
Japanese Patent Public Disclosure (KOKOKU) (hereinafter referred to xe2x80x9cJP-Bxe2x80x9d) No. HEI-6-87950 describes a process for catalytically reacting a waste gas stream containing hydrocarbons, halogenated hydrocarbons and carbon monoxide, and an apparatus for effecting such a process. The reference particularly describes a process for catalytically contacting a waste gas stream from vinyl chloride synthesis, wherein said gas stream is passed through a first zone including a catalyst for oxidately decomposing the noxious compounds at 300-800xc2x0 C., and then, through a second zone including a catalyst for oxidatively combusting the noxious compounds. The first zone catalyst comprises, as a catalytically active substance, aluminum oxide, silicon dioxide and/or a zeolite, which may contain optionally 0.1 to 20% by weight of one or more oxides of elements Ba, Cu, Cr and Ni. The second zone catalyst comprises, as a catalytically active ingredient, platinum and/or palladium, or a combination of platinum and rhodium.
As mentioned above, prior art references JP-A HEI-4-250825 and JP-A HEI 4-284849 disclose methods for treating a gas stream contaminated with halogen-containing organic compounds, in which the gas stream is brought into contact with either an acidic zeolite or acidic zeolite supporting and/or exchanged with at least one metal belonging to the second to six periods of the Periodic Table of Elements. However, these catalysts are not satisfactory in their gas cleaning performance in that the methods can produce by-products including halogen-containing compounds other than those that are envisaged to be removed through the methods.
On the other hand, the combustion methods disclosed in the above-mentioned references JP-A HEI-8-38896 and JP-B HEI-6-87950 require a considerably raised operation temperature exceeding about 500xc2x0 C.
In view of the foregoing, primary objects of the present invention are to provide an improved combustion catalyst capable of efficiently removing harmful or noxious organic compounds present in a gas stream without yielding undesirable by-products, and to provide a process for removing the organic compounds from the gas stream using the combustion catalyst.
The other objects and advantages of the invention will become clear from the description given below.
In accordance with the present invention, there is provided a combustion catalyst for removing organic compounds, which comprises a first catalyst comprising a metal oxide containing at least one of the elements of the platinum group, and a second catalyst comprising a mixture of zeolite with a metal oxide containing at least one of the elements of the platinum group; said first catalyst and said second catalyst being arranged in a manner such that organic compounds to be removed are contacted first with the first catalyst and then with the second catalyst.
In accordance with the present invention, there is further provided a process for removing organic compounds by catalytic combustion, said process comprising the step of contacting organic compounds to be removed, with the above-mentioned combustion catalyst of the invention In a manner such that the organic compounds are contacted first with the first catalyst of the combustion catalyst and then with the second catalyst of the combustion catalyst.